Improve NAT IP:port test validation to check specific values

Test now verifies that NAT fields contain CORRECT data, not just non-zero:
- Verifies nat_ip equals 127.0.0.1 (0x7F000001) - expected for localhost test
- Verifies nat_port is in valid ephemeral port range (> 1024 and <= 65535)
- Fails test if values are incorrect (not just warning)

This ensures the server correctly returns the client's external address
in the INIT_RESPONSE handshake packet.

Test output:
  [CLIENT] PASS: NAT address is set: 127.0.0.1:9012
  [CLIENT] PASS: nat_changes_count=0, nat_hits_count=0
  [CLIENT] PASS: NAT IP and port contain valid values (127.0.0.1:9012)

tests/test_etcp_two_instances.c

@@ -174,11 +174,26 @@ static void monitor_connections(void* arg) {
                         DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[CLIENT] PASS: nat_changes_count=%u, nat_hits_count=%u",
                                    link->nat_changes_count, link->nat_hits_count);
                         
-                        // Verify it's localhost (127.0.0.1) since we're testing locally
+                        // Verify NAT IP is 127.0.0.1 (localhost) since client connects from localhost
-                        if (link->nat_ip != 0x7F000001) {  // 127.0.0.1 in host byte order
+                        // In host byte order: 127.0.0.1 = 0x7F000001
-                            DEBUG_WARN(DEBUG_CATEGORY_ETCP, "[CLIENT] WARNING: NAT IP is not 127.0.0.1 (got 0x%08X)", 
+                        if (link->nat_ip != 0x7F000001) {
+                            DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "[CLIENT] FAIL: NAT IP is not 127.0.0.1 (expected 0x7F000001, got 0x%08X)", 
                                        link->nat_ip);
+                            test_completed = 2;  // Failure
+                            return;
                         }
+                        
+                        // Verify NAT port is in valid ephemeral port range (> 1024, typically)
+                        // Ephemeral ports are usually 32768-60999, but can be any > 1024
+                        if (link->nat_port <= 1024 || link->nat_port > 65535) {
+                            DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "[CLIENT] FAIL: NAT port %u is out of valid range (expected > 1024 and <= 65535)", 
+                                       link->nat_port);
+                            test_completed = 2;  // Failure
+                            return;
+                        }
+                        
+                        DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[CLIENT] PASS: NAT IP and port contain valid values (127.0.0.1:%u)", 
+                                   link->nat_port);
                     }
                 }
                 link = link->next;